Ofdm-cdma equipment and method

ABSTRACT

An OFDM signal transmission apparatus is provided, which includes a mapping unit configured to map first signals into N subcarriers and second signals into M subcarrier(s) to form an OFDM signal, wherein N is larger than M. The first signals are each indicating a same bit of retransmission information and the second signals are each indicating a same bit of information other than retransmission information. The OFDM signal transmission apparatus further includes a transmitting unit configured to transmit the formed OFDM signal.

BACKGROUND

1. Technical Field

The present invention relates to a transmission/reception apparatus usedfor digital mobile communications, and particularly relates to an OFDM(Orthogonal Frequency Division Multiplexing) transmission/receptionapparatus and OFDM transmission/reception method used for OFDM-basedmobile communications.

2. Description of the Related Art

In an OFDM-based mobile communication, transmission data is converted toa plurality of slower parallel signals, which are superimposed overtheir respective subcarriers and transmitted. The following is anexplanation of a conventional OFDM transmission/reception apparatus withreference to FIG. 1 and FIG. 2. FIG. 1 is a block diagram showing aconfiguration of a transmission system of a conventional OFDMtransmission/reception apparatus. FIG. 2 is a block diagram showing aconfiguration of a reception system of the conventional OFDMtransmission/reception apparatus. For simplification, suppose aplurality of signal lines are also expressed with a single arrow.

In FIG. 1, modulation section 11 modulates transmission data. InverseFast Fourier Transform (hereinafter referred to as “IFFT”) section 12performs IFFT processing on the modulated transmission data.

Transmission section 13 performs transmission processing on theIFFT-processed transmission data and transmits this data from antenna14. In FIG. 2, antenna 21 receives a radio signal.

Reception section 22 performs reception processing on the signalreceived from antenna 21 (reception signal). Timing control section 23controls each section according to a symbol synchronization timingacquired from reception section 22. Fast Fourier Transform (hereinafterreferred to as “FFT) section 24 performs FFT processing on the receptionsignal. Regarding the acquisition of symbol synchronization timing inreception section 22, a variety of methods are proposed. However, theirdetailed explanations are omitted here.

Synchronization detection section 25 performs synchronization detectionprocessing on the FFT-processed reception signal and removes influencesof phase rotation and amplitude variation caused by fading, etc. fromthe reception signal above.

Thus, the conventional OFDM transmission/reception apparatus achieveslarge-capacity and high-quality radio communications with excellentmulti-path adaptability through data transmission/reception using aplurality of subcarriers.

However, in the conventional OFDM transmission/reception apparatus,important information used for communication control such as controlinformation and retransmission information (hereinafter, simply referredto as “important information”) is multiplied by a carrier frequencysignal together with user data and transmitted through one ofsubcarriers.

Therefore, if the reception quality of subcarriers carrying theimportant information deteriorates in an extreme manner due toinfluences of fading, etc., errors may occur in bits representingcontrol information and retransmission information, which will preventappropriate control and retransmission requests, etc., extremelydeteriorating the channel quality.

BRIEF SUMMARY

The present invention has been implemented taking account of the pointsdescribed above and it is an objective of the present invention toprovide an OFDM transmission/reception apparatus capable of improvingthe quality of important information received by the apparatus on thereceiving side.

This objective is achieved by transmitting important information withtwo subcarriers instead of one subcarrier in the conventional manner.This objective is further achieved by using one of the two subcarrierscarrying important information as the subcarrier with a carrierfrequency signal (DC signal) of frequency 0 which was conventionally notused.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The above and other objects and features of the invention will appearmore fully hereinafter from a consideration of the following descriptiontaken in connection with the accompanying drawing wherein one example isillustrated by way of example, in which;

FIG. 1 is a block diagram showing a configuration of a transmissionsystem of a conventional OFDM transmission/reception apparatus;

FIG. 2 is a block diagram showing a configuration of a reception systemof the conventional OFDM transmission/reception apparatus;

FIG. 3A is a schematic diagram showing a spectrum example in theconventional OFDM system;

FIG. 3B is a schematic diagram showing a spectrum example according toEmbodiment 1 of the present invention;

FIG. 4 is a block diagram showing a configuration of a transmissionsystem of an OFDM transmission/reception apparatus according toEmbodiment 1 of the present invention;

FIG. 5 is a block diagram showing a configuration of a reception systemof the OFDM transmission/reception apparatus according to Embodiment 1of the present invention;

FIG. 6 is a block diagram showing a configuration of a synchronizationdetection section in the reception system of the OFDMtransmission/reception apparatus according to Embodiment 1 of thepresent invention;

FIG. 7 is a block diagram showing a configuration of a reception systemof an OFDM transmission/reception apparatus according to Embodiment 2 ofthe present invention;

FIG. 8 is a block diagram showing a configuration of a reception systemof an OFDM transmission/reception apparatus according to Embodiment 3 ofthe present invention;

FIG. 9 is a block diagram showing a configuration of a reception systemof an OFDM transmission/reception apparatus according to Embodiment 4 ofthe present invention;

FIG. 10 is a block diagram showing a configuration of a synchronizationdetection section in the reception system of the OFDMtransmission/reception apparatus according to Embodiment 4 of thepresent invention;

FIG. 11 is a block diagram showing a configuration of a reception systemof an OFDM transmission/reception apparatus according to Embodiment 5 ofthe present invention;

FIG. 12 is a block diagram showing a configuration of a reception systemof an OFDM transmission/reception apparatus according to Embodiment 6 ofthe present invention;

FIG. 13 is a block diagram showing a configuration of a reception systemof an OFDM transmission/reception apparatus according to Embodiment 7 ofthe present invention;

FIG. 14 is a block diagram showing a configuration of a combinationsection of the reception system of the OFDM transmission/receptionapparatus according to Embodiment 7 of the present invention;

FIG. 15 is a schematic diagram showing a spectrum example of an OFDMtransmission/reception apparatus according to Embodiment 8 of thepresent invention; and

FIG. 16 is a block diagram showing a configuration of a reception systemof the OFDM transmission/reception apparatus according to Embodiment 8of the present invention.

DETAILED DESCRIPTION

With reference now to the attached drawings, the embodiments of thepresent invention are explained in detail below.

Embodiment 1

The OFDM transmission/reception apparatus according to the presentembodiment transmits important information with two subcarriers and usesone of these two subcarriers carrying important information to carry acarrier frequency signal (DC signal) of angular frequency 0 which wasconventionally not used.

A conceivable method to prevent the reception quality of a subcarriercontaining important information from extremely deteriorating due toinfluences of fading, etc. causing errors in the bits representingcontrol information and retransmission information is to carry sameimportant information through a plurality of subcarriers and for theapparatus on the receiving side to use the important information carriedby the subcarrier with the best reception quality or combine allimportant information carried.

This method may indeed improve the reception quality of importantinformation at the apparatus on the receiving side, however causes a newproblem of increasing the amount of transmission data in addition to theuser data, thus reducing transmission efficiency.

Therefore, the present embodiment will transmit same importantinformation with two subcarriers and use, as one of these twosubcarriers, a subcarrier with a carrier frequency signal (DC signal;hereinafter simply referred to as “DC”) of frequency 0 which wasconventionally not used as a subcarrier, thus preventing deteriorationof transmission efficiency.

As shown in a spectrum diagram of FIG. 3A, a normal OFDM system uses aneven number of subcarriers and the subcarrier with DC shown by a dottedline in the diagram is not used as a subcarrier. FIG. 3A shows thatimportant information is mapped onto the second subcarrier indicated byhatching.

Thus, the present embodiment transmits important information that usedto be transmitted with one subcarrier (here, the second subcarrier) witha DC subcarrier, which is conventionally not used as a subcarrier, andin this way the present embodiment can transmit same importantinformation through two subcarriers without deteriorating thetransmission efficiency.

FIG. 3B shows a spectrum diagram when same important information iscarried by two subcarriers in the present embodiment. As is clear fromFIG. 3B, the same important information is multiplied not only by thesecond subcarrier but also by the DC subcarrier.

The OFDM transmission/reception apparatus according to the presentembodiment is explained with reference to FIG. 4 through FIG. 6 below.FIG. 4 is a block diagram showing a configuration of a transmissionsystem of the OFDM transmission/reception apparatus according toEmbodiment 1 of the present invention. FIG. 5 is a block diagram showinga configuration of a reception system of the OFDM transmission/receptionapparatus according to Embodiment 1 of the present invention. FIG. 6 isa block diagram showing a configuration of a synchronization detectionsection in the reception system of the OFDM transmission/receptionapparatus according to Embodiment 1 of the present invention. In thepresent embodiment, important information is transmitted with twosubcarriers; the second subcarrier and DC subcarrier, as shown in FIG.3B.

In FIG. 4, modulation section 201 performs modulation processing ontransmission data. Mapping control section 202 controls IFFT section 203so that the important information is mapped onto the second subcarrierand DC subcarrier. IFFT section 203 performs IFFT processing on themodulated transmission data. Transmission section 204 performstransmission processing on the IFFT-processed transmission data andtransmits the transmission-processed transmission data from antenna 205.

In FIG. 5, antenna 301 receives a radio signal. Reception section 302performs reception processing on the signal received by antenna 301(reception signal). Timing control section 303 controls each sectionaccording to a symbol synchronization timing acquired from receptionsection 302. FFT section 304 performs FFT processing on the receptionsignal. Regarding the acquisition of symbol synchronization timing atreception section 302, a variety of methods are proposed. The presentembodiment can use any method for the acquisition of symbolsynchronization timing.

Synchronization detection section 305 performs synchronization detectionprocessing (demodulation processing) on the FFT-processed receptionsignal and removes influences of phase rotation and amplitude variationcaused by fading, etc. from the reception signal above.

Important information extraction section 306 extracts importantinformation from the reception signal subjected to synchronizationdetection processing important information selection section 307 detectsthe subcarrier with the higher reception level of the two subcarrierswith important information multiplied based on reception levelinformation detected by synchronization detection section 305, andextracts and outputs the important information carried by the detectedsubcarrier.

In FIG. 6 showing the synchronization detection section, based on thesymbol synchronization timing indicated by timing control section 303,known symbol extraction section 401 extracts a signal during a knownsymbol interval from the FFT-processed reception signal to output thesignal to multiplier 402, and extracts a signal during a data symbolinterval to output the signal to calculation section 407.

Multiplier 402 multiplies the signal of the known symbol interval of theFFT-processed reception signal by a predetermined known symbol andcalculates phase rotation and amplitude variation of the receptionsignal influenced by fading, etc.

Sum-of-squares calculation section 403 calculates a sum of squares of anI component and Q component of the output of known symbol extractionsection 401. The calculated sum of squares is output to divider 404 androot calculator 408.

Divider 404 divides the output of multiplier 402 by the output ofsum-of-squares calculation section 403. Memory 405 temporarily storesthe output of divider 404.

Switch 406 outputs the output of divider 404 stored in memory 405 tocalculation section 407 while the signal during the data symbol intervalof the reception signal is being input to calculation section 407 basedon the symbol synchronization timing indicated by timing control section303.

Calculation section 407 generates a conjugate complex number of theoutput of divider 404, multiplies the signal during the data symbolinterval of the reception signal by this conjugate complex number toobtain a synchronization detection signal.

Root calculator 408 carries out a root calculation on the sum of squaresfrom sum-of-squares calculation section 403 and calculates the receptionlevel of the reception signal. The calculated reception level is outputto important information selection section 307.

Then, the operation of the OFDM transmission/reception apparatus in sucha configuration is explained below. According to FIG. 4, thetransmission data is modulated by modulation section 201. While beingcontrolled by mapping control section 202 so that important informationis multiplied by the second subcarrier and DC subcarrier, the modulatedtransmission data is IFFT-processed by IFFT section 203. TheIFFT-processed transmission data is subjected to transmission processingin transmission section 204 and then transmitted from antenna 205.

The radio signal with the important information multiplied by the secondsubcarrier and DC subcarrier is received by antenna 301 (see FIG. 5).The signal received by antenna 301 (reception signal) is subjected toreception processing by reception section 302. The reception-processedreception signal is FFT-processed by the FFT section 304. TheFFT-processed reception signal is subjected to synchronization detectionprocessing by synchronization detection section 305.

In important information extraction section 306, the importantinformation carried by the DC subcarrier and the important informationcarried by the second subcarrier are extracted from the reception signalsubjected to synchronization detection processing. Each extractedimportant information is output to important information selectionsection 307. In important information selection section 307, of theimportant information extracted by important information extractionsection 306, the important information carried by the subcarrier withthe higher reception level is output, based on the reception levelinformation output from synchronization detection section 305.

Thus, according to the present embodiment, the apparatus on thetransmitting side carries important information with two subcarriers andthe apparatus on the receiving side uses, of the important informationcarried by the two subcarriers, the important information with thebetter reception level, which allows the reception quality of importantinformation to be maintained even in a fading environment where only thereception level of a specific subcarrier deteriorates.

Furthermore, when transmitting important information with twosubcarriers, the present embodiment uses, as one of the subcarriers, theDC subcarrier, which was conventionally not used as a subcarrier, and inthis way can transmit important information with two subcarriers withoutreducing the transmission efficiency.

The number of subcarriers carrying same important information is notlimited to 2 as described above; it goes without saying that the qualityof important information will further be improved by using moresubcarriers and selecting ones with better reception conditions.However, assigning many subcarriers to important information willdecrease the number of subcarriers used for transmission of user dataand reduce the transmission efficiency, and therefore it is mostdesirable to limit the number of subcarriers carrying importantinformation to 2 and use as one of them a DC subcarrier as in the caseof the present embodiment.

Embodiment 2

The OFDM transmission/reception apparatus according to the presentembodiment has the same configuration as that of the OFDtransmission/reception apparatus according to Embodiment 1 except thatthe reception level, one of outputs of the synchronization detectionsection, is averaged.

The OFDM transmission/reception apparatus according to the presentembodiment is explained with reference to FIG. 7. FIG. 7 is a blockdiagram showing a configuration of a reception system of the OFDMtransmission/reception apparatus according to Embodiment 2 of thepresent invention. The parts in FIG. 7 with the same configuration asthat of Embodiment 1 (FIG. 5) are assigned the same codes as those inFIG. 5 and their detailed explanations are omitted. The block diagram ofthe transmission system is omitted. The present embodiment also uses thesecond subcarrier as the subcarrier other than the DC subcarrier tocarry important information.

In FIG. 7, averaging section 501 averages reception level information,one of the outputs of synchronization detection section 305 and outputsthe averaged reception level information to important informationselection section 307.

Thus, when comparing the reception level of subcarriers carryingimportant information, the present embodiment uses the averagedreception level, and in this way can improve the accuracy incomparing/determining the reception level and thus improve the receptionquality of important information. In addition, the number of slots to beaveraged or time interval can be determined arbitrarily.

Embodiment 3

The OFDM transmission/reception apparatus according to the presentembodiment has the same configuration as that of the OFDMtransmission/reception apparatus according to Embodiment 2 except thatwhen comparing the reception level of the DC subcarrier and secondsubcarrier carrying important information, if the reception level of theDC subcarrier is higher, the important information carried by the DCsubcarrier is used only when the difference in the reception levelbetween the DC subcarrier and the second subcarrier exceeds apredetermined value.

Since a DC offset may be carried on a signal carried by the DCsubcarrier in analog circuits of the apparatuses on the transmittingside and on the receiving side, the reception level at the apparatus onthe receiving side may include an error corresponding to the DC offsetcomponent.

Therefore, even if the result of a comparison of the reception levelbetween the DC subcarrier and second subcarrier shows that the receptionlevel of the DC subcarrier is higher, if the difference in the receptionlevel between the DC subcarrier and second subcarrier is small, thereception level of the DC subcarrier with the DC offset componentremoved may fall below the reception level of the second subcarrier.

Therefore, the present embodiment uses a threshold greater than aconceivable DC offset, and uses the important information carried by theDC subcarrier only when it is determined that the difference in thereception level between the DC subcarrier and the second subcarrier issufficiently greater than the DC offset.

The OFDM transmission/reception apparatus according to the presentembodiment is explained with reference to FIG. 8. FIG. 8 is a blockdiagram showing a configuration of a reception system of the OFDMtransmission/reception apparatus according to Embodiment 3 of thepresent invention. The parts in FIG. 8 with the same configuration asthat of Embodiment 2 (FIG. 7) are assigned the same codes as those inFIG. 7 and their detailed explanations are omitted. The block diagram ofthe transmission system is omitted. The present embodiment also uses thesecond subcarrier as the subcarrier other than the DC subcarrier tocarry important information.

In FIG. 8, subtractor 601 performs a subtraction between the receptionlevel of the DC subcarrier and the reception level of the secondsubcarrier. Subtractor 602 compares a threshold and the subtractionresult, which is the output of subtractor 601.

Determination section 603 determines whether the output of subtractor602 is positive or negative, determines whether the difference in thereception level between the DC subcarrier and the second subcarrierexceeds the threshold or not and outputs the determination result toimportant information selection section 307.

Based on the determination result, important information selectionsection 307 outputs the important information carried by the DCsubcarrier when the difference in the reception level between the DCsubcarrier and the second subcarrier exceeds the threshold, and outputsthe important information carried by the second subcarrier when thedifference in the reception level between the DC subcarrier and thesecond subcarrier is smaller than the threshold.

In this way, the present embodiment can compare/determine the receptionlevel taking account of influences of a DC offset by comparing thedifference in the reception level between the DC subcarrier and thesecond subcarrier. This improves the accuracy in comparing/determiningthe reception level, thus improving the reception quality of importantinformation.

Embodiment 4

The OFDM transmission/reception apparatus according to the presentembodiment has the same configuration as that of the OFDMtransmission/reception apparatus according to Embodiment 2 except that adetermination error, instead of the reception level, is used to select asubcarrier.

As mentioned above, the signal carried by the DC subcarrier contains anerror called “DC offset”, and therefore the reception quality of the DCsubcarrier may be deteriorated more than other subcarriers. Thus, thepresent embodiment uses a determination error instead of the receptionlevel when selecting a subcarrier.

The OFDM transmission/reception apparatus according to the presentembodiment is explained with reference to FIG. 9 and FIG. 10. FIG. 9 isa block diagram showing a configuration of a reception system of theOFDM transmission/reception apparatus according to Embodiment 4 of thepresent invention. FIG. 10 is a block diagram showing a configuration ofa synchronization detection section of the reception system of the OFDMtransmission/reception apparatus according to Embodiment 4 of thepresent invention. The parts in FIG. 9 and FIG. 10 with the sameconfiguration as that of Embodiment 1 or Embodiment 2 are assigned thesame codes as those in Embodiment 1 or Embodiment 2 and their detailedexplanations are omitted. The present embodiment also uses the secondsubcarrier as the subcarrier other than the DC subcarrier to carryimportant information.

In FIG. 9, synchronization detection section 701 outputs determinationerrors of the DC subcarrier and the second subcarrier to averagingsection 501. Averaging section 501 calculates an average value of thesedetermination errors and outputs the average value to importantinformation selection section 307. Important information selectionsection 307 selects and outputs the important information carried by thesubcarrier with the smaller determination error.

In FIG. 10, determination section 801 determines a synchronizationdetection signal. Subtractor 802 performs a subtraction before and afterthe determination of the signal, generates determination errors andoutputs the generated determination errors to averaging section 501.

The calculated determination errors are averaged by averaging section501. This generates an average value of the determination errors. Theaverage value of the determination errors is output to importantinformation selection section 307.

Important information selection section 307 compares the determinationerror of the DC subcarrier and that of the second subcarrier and selectsand outputs the important information carried by the subcarrier with thesmaller determination error.

Thus, the present embodiment determines which subcarrier has betterreception quality based on the determination error of the DC subcarrierand that of the second subcarrier to ignore influences of a DC offsetand in this way can improve the reception quality of importantinformation.

Embodiment 5

The OFDM transmission/reception apparatus according to the presentembodiment has the same configuration as that of the OFDMtransmission/reception apparatus according to Embodiment 1 except thatthe FFT-processed DC subcarrier signal is stripped of a DC offset andthen subjected to synchronization detection processing.

A DC offset has a constant value irrespective of its digital signalwaveform and the probability that a digital signal will take a value of1 or 0 is assumed to be 50%. Therefore, if FFT-processed digital signalsare added up and averaged, 1 and 0 are canceled out and only the DCoffset value can be detected. Thus, the present embodiment eliminatesthe DC offset component from the DC subcarrier signal beforesynchronization detection processing according to the method above.

The apparatus according to the present embodiment is explained withreference to FIG. 11. FIG. 11 is a block diagram showing a configurationof a reception system of the OFDM transmission/reception apparatusaccording to Embodiment 5 of the present invention. The parts in FIG. 11with the same configuration as that of Embodiment 4 are assigned thesame codes and their detailed explanations are omitted. The blockdiagram of the transmission system is omitted. The present embodimentalso uses the second subcarrier as the subcarrier other than the DCsubcarrier to carry important information.

In FIG. 11, averaging section 901 adds up and averages the FFT-processedDC subcarrier signals. This calculated average value is a DC offsetvalue. The number of slots added up or the time interval can bedetermined arbitrarily. Memory 902 stores the calculated DC offsetvalue.

Subtractor 903 reads a DC offset value in memory 902 one by one andsubtracts the read DC offset value from the FFT-processed DC subcarriersignal. This makes it possible to eliminate the DC offset from the DCsubcarrier signal.

Moreover, when removing a DC offset from the FFT-processed signal inslot n, if the DC offset value calculated from the signal in slot n isused, the DC offset eliminating speed is slowed down. Thus, it isdesirable to use a DC offset calculated from a few immediately precedingslots taking account of the fact that DC offsets are considered to bealmost constant when focused on a unit slot for a long period of time.

For example, averaging section 901 calculates a DC offset value forslots n-3 to n-1 and subtractor 903 subtracts this DC offset value fromthe reception signal in slot n. In this way, using a DC offset value fora few immediately preceding slots allows DC offset eliminationprocessing without time lags.

Thus, the present embodiment adds up and averages DC subcarrier signalsto calculate a DC offset and eliminates this DC offset from the DCsubcarrier signals, which improves the reception quality of importantinformation.

Embodiment 6

The OFDM transmission/reception apparatus according to the presentembodiment has the same configuration as that of the OFDMtransmission/reception apparatus according to Embodiment 5 except thatimportant information carried by the DC subcarrier and importantinformation carried by the second subcarrier are combined.

Embodiments 1 through 5 describe embodiments that select the DCsubcarrier or second subcarrier, whichever has a better receptioncondition. However, taking account of the fact that the receptionquality of the DC subcarrier is also improved by eliminating DC offsets,the present embodiment combines both to obtain important information.

The OFDM transmission/reception apparatus according to the presentembodiment is explained with reference to FIG. 12. FIG. 12 is a blockdiagram showing a configuration of a reception system of the OFDMtransmission/reception apparatus according to Embodiment 6 of thepresent invention. The parts in FIG. 12 with the same configuration asthat of Embodiment 1 are assigned the same codes and their detailedexplanations are omitted. The block diagram of the transmission systemis omitted. The present embodiment also uses the second subcarrier asthe subcarrier other than the DC subcarrier to carry importantinformation.

In FIG. 12, combination section 1001 combines the important informationcarried by DC subcarrier and the second subcarrier which are extractedby important information extraction section 306, and outputs thecombined important information.

Thus, the present embodiment eliminates influences of DC offsets fromthe DC subcarrier signal and then combines the important informationcarried by the two subcarriers, which improves the reception quality ofimportant information.

Embodiment 7

The OFDM transmission/reception apparatus according to the presentembodiment has the same configuration as that of the OFDMtransmission/reception apparatus according to Embodiment 6 except thatcombination processing is carried out after weighting processingaccording to the reception level is carried out.

The OFDM transmission/reception apparatus according to the presentembodiment is explained with reference to FIG. 13 and FIG. 14. FIG. 13is a block diagram showing a configuration of a reception system of theOFDM transmission/reception apparatus according to Embodiment 7 of thepresent invention. FIG. 14 is a block diagram showing a configuration ofa combination section of the reception system of the OFDMtransmission/reception apparatus according to Embodiment 7 of thepresent invention. The parts in FIG. 13 and FIG. 14 with the sameconfiguration as that of Embodiment 6 are assigned the same codes asthose in Embodiment 6 and their detailed explanations are omitted. Thepresent embodiment also uses the second subcarrier as the subcarrierother than the DC subcarrier to carry important information.

In FIG. 13, reception level information, which is one of outputs ofsynchronization detection section 305, is input to combination section1101. Combination section 1101 performs a maximum-ratio combination onimportant information carried by the DC subcarrier and secondsubcarrier. That is, combination section 1101 carries out weightingprocessing on each important information and then combines theweighting-processed important information.

In FIG. 14, multiplier 1201 multiplies the important information carriedby the DC subcarrier which are extracted by important informationextraction section 306 by the reception level information of the DCsubcarrier calculated by synchronization detection section 305.

Likewise, multiplier 1202 multiplies the important information carriedby the second subcarrier which are extracted by important informationextraction section 306 by the reception level information of the secondsubcarrier calculated by synchronization detection section 305. Adder1203 adds up the output of multiplier 1201 and the output of multiplier1202 and combines weighting-processed important information.

Thus, the present embodiment carries out a combination reflecting thereception level by eliminating influences of DC offsets from the DCsubcarrier signal and then performing a maximum-ratio combination on theimportant information carried by the two subcarriers. In this way, thepresent embodiment can improve the reception quality of importantinformation more than Embodiment 6.

Embodiment 8

The OFDM transmission/reception apparatus according to the presentembodiment applies a specific packet (for example, packet carryingcontrol information, packet carrying retransmission information orpacket for a user with poor channel quality, etc.) as importantinformation carried by a plurality of subcarriers in Embodiments 1through 7. Here, the OFDM transmission/reception apparatus according tothe present embodiment is explained with reference to Embodiment 1, butthe OFDM transmission/reception apparatus according to the presentembodiment is also applicable to Embodiments 2 to 7.

The OFDM transmission/reception apparatus according to the presentembodiment is explained with reference to FIG. 15 and FIG. 16. FIG. 15is a schematic diagram showing a spectrum example of the OFDMtransmission/reception apparatus according to Embodiment 8 of thepresent invention. FIG. 16 is a block diagram showing a configuration ofa reception system of the OFDM transmission/reception apparatusaccording to Embodiment 8 of the present invention. The transmissionsystem of the OFDM transmission/reception apparatus according to thepresent embodiment is mostly the same as that shown in FIG. 4.

In FIG. 4, which illustrates the transmission system, a transmissionsignal including a specific packet is modulated by modulation section201. This specific packet is, for example, a signal transmitted througha control channel or a signal transmitted through multicast channels(channels received by a plurality of users). Needless to say, it is alsopossible to use a packet of a predetermined user, such as packet of auser having a reception system with poor reception quality, etc.

Mapping control section 202 controls IFFT section 203 so that thespecific packet in the modulated transmission signal is mapped onto aplurality of subcarriers. The concrete mapping method by mapping controlsection 202 is as follows:

In mapping control section 202, data 1 of the modulated specific packetis mapped, for example, onto two subcarriers, subcarrier #1 andsubcarrier #1′ shown in FIG. 15. Furthermore, data 2 of the modulatedspecific packet is mapped, for example, onto two subcarriers, subcarrier#2 and subcarrier #2′ shown in FIG. 15. Likewise, each piece of data ofthe modulated specific packet is mapped onto either one of the twosubcarriers shown in FIG. 15.

Here, the case where there are two subcarriers onto which each piece ofdata of the modulated specific packet is mapped is explained, but thenumber of subcarriers above can further be increased. The number ofsubcarriers above can be determined according to various conditions suchas transmission efficiency, etc.

Furthermore, mapping control section 202 controls IFFT section 203 sothat packets other than the specific packet above of the modulatedtransmission signal is mapped onto one subcarrier as in the case of theconventional method.

The modulated transmission signal is controlled by mapping controlsection 202 as shown above and subjected to IFFT processing by IFFTsection 203. The IFFT-processed transmission signal is subjected totransmission processing by transmission section 204 and then transmittedfrom antenna 205.

The radio signal with each piece of data in a specific packet mapped onany two subcarriers is received by antenna 1401 in FIG. 16. The signalreceived by antenna 1401 (reception signal) is subjected topredetermined reception processing and then FFT processing by FFTsection 1402. The FFT-processed reception signal is subjected tosynchronization detection processing by synchronization detectionsection 1403. Here, synchronization detection processing throughsynchronization detection section 1403 can be replaced by delaydetection processing. The reception signal subjected to synchronizationdetection processing (or delay detection processing) is sent toselection section 1404.

The specific packet of the reception signal above is sent from selectionsection 1404 to diversity section 1405. In diversity section 1405,diversity reception processing is performed. That is, of the data piecesmapped on two subcarriers of the specific packet, the data piece withthe higher reception level is selected or both data pieces mapped on thetwo subcarriers of the specific packet are combined.

As shown above, according to the present embodiment, the apparatus onthe transmitting side carries data in a specific packet with a pluralityof subcarriers and the apparatus on the receiving side selects the datapiece with the better reception level of those data pieces of thespecific packet above or combines the data pieces in the specific packetabove, which allows the reception quality of the specific packet to bemaintained even in a fading environment where only the reception levelof a specific subcarrier deteriorates. This makes it possible to improvethe error rate characteristic of the specific packet without reducingthe transmission efficiency.

As explained above, the present invention transmits importantinformation with two subcarriers instead of one subcarrier in theconventional method and uses a DC subcarrier as one of the twosubcarriers carrying important information, thus can improve thereception quality of the important information at the apparatus on thereceiving side without reducing the transmission efficiency.

The present invention is not limited to the above described embodiments,and various variations and modifications may be possible withoutdeparting from the scope of the present invention.

This application is based on the Japanese Patent Application No. HEI11-054667 filed on Mar. 2, 1999 and the Japanese Patent Application No.HEI 11-249938 filed on Sep. 3, 1999, entire content of which isexpressly incorporated by reference herein.

1. An OFDM signal transmission apparatus comprising: a mapping controlsection configured to map first modulated signals corresponding to data1 of a specific packet into a first number of subcarriers and to mapsecond modulated signals corresponding to data 2 of a packet other thanthe specific packet into a second number of subcarriers to form an OFDMsignal, the specific packet including retransmission information, thefirst number of the first modulated signals including the same data asthe data 1, and the second number of the second modulated signalsincluding the same data as the data 2; and a transmission sectionconfigured to transmit the formed OFDM signal, wherein the first numberis larger than the second number.
 2. The OFDM signal transmissionapparatus according to claim 1, wherein the second number equals one. 3.An OFDM signal transmission method comprising: mapping first modulatedsignals corresponding to data 1 of a specific packet into a first numberof subcarriers and second modulated signals corresponding to data 2 of apacket other than the specific packet into a second number ofsubcarriers to form an OFDM signal, the specific packet includingretransmission information, the first number of the first modulatedsignals including the same data as the data 1, and the second number ofthe second modulated signals including the same data as the data 2; andtransmitting the formed OFDM signal, wherein the first number is largerthan the second number.
 4. The OFDM signal transmission method accordingto claim 3, wherein the second number equals one.